1. Field of the Invention
This invention relates generally to detecting defective semiconductor integrated circuit wafers during fabrication thereof, and in particular, to an apparatus and method that detects burnt material on the surface of the integrated circuit wafer by the lack of light reflectance therefrom.
2. Description of the Related Technology
The semiconductor integrated circuit has revolutionized the field of electronics by becoming a basic building block in electronic products. The integrated circuit has created many new products and services in the consumer, industrial and military markets. As new products were introduced, the more popular ones created a great demand for the integrated circuits used to build these products.
This resulted in a new industry devoted to manufacturing semiconductor integrated circuits in quantity and at a price that was acceptable in the various electronic markets. In order to keep the price of integrated circuits as low as possible, assembly line, automation and large quantity manufacturing techniques were required.
Semiconductor integrated circuits are manufactured or fabricated by creating electronic circuits such as transistors, diodes, resistors and capacitors on a silicon substrate. This silicon substrate is a wafer cut from a pure silicon crystal grown specifically for use in manufacturing integrated circuits. The silicon wafer is normally cut to resemble a thin circular disk of about 4 to 6 inches in diameter. The wafer is smoothly polished to a mirror-like surface and then the fabrication process of creating the electronic circuits begins.
The fabrication process begins by selectively doping the surface area of the silicon wafer with impurities to create negative channel (N--channel) or positive channel (P--channel) regions to create transistors and diodes. Interconnections, resistors and capacitors are fabricated by selectively plating metal onto the surface of the silicon wafer. Insulation between layers of electronic components is fabricated by selectively coating the areas of the wafer requiring insulation.
An integrated circuit wafer is comprised of a plurality of integrated circuit chips that ultimately are fabricated into integrated circuit packages for use in building electronic products. Thus, during integrated circuit chip fabrication, a plurality of duplicate fabrication operations are performed on the silicon wafer.
These fabrication operations consist of applying photo resist to the surface of the wafer and selectively etching parts of the silicon wafer to produce the required electronic circuits. Integrated circuit fabrication involves a plurality of steps that layer specific patterns of protective coatings onto the surface of the wafer, then doping, etching or depositing metal at the unprotected areas of the wafer. After the coating, doping, etching and depositing of metal steps are completed, The wafer is cut into individual integrated circuit chips for packaging into finished integrated circuit products.
A necessary step in the aforementioned fabrication process is depositing and curing the protective coatings or "resist" onto the wafer surface. The resist is cured with heat, and sometimes, if the process parameters are not adjusted correctly or the process equipment malfunctions, excessive heat will discolor or "burn" the resist surface. In similar fashion, coatings of organic material such as, for example, polymers including polyimide and epoxy may be used on the surface of the integrated circuit wafer die. Burnt resist or mask is not desirable because it introduces contaminates into and causes other problems in the integrated circuit fabrication process.
Detection of burnt resist or mask was not made until wafer etch inspection, resulting in the loss of many wafers. The number of wafers lost depends on the fabrication lag time from resist cure to etch inspection. With highly automated integrated circuit fabrication assembly lines, the number of undetected burnt resist or mask wafers can be enormous. In addition, as integrated circuits become more complex and expensive, the cost in spoilage becomes a significant part of the overall manufacturing costs. This is especially true when small quantity production run, or custom order application-specific integrated circuits (ASIC) are being produced.
What is needed in semiconductor integrated circuit manufacturing is more timely detection of burnt resist or mask during the wafer fabrication process. Preferably some form of automatic detection of burnt resist would be desirable so that the wafer manufacturing process could quickly be shut down while corrective adjustments to the process are made.